Uniquely human brain region enables punishment decisions

Humans are unique among social creatures in their willingness to bear personal costs to punish those who have harmed others. A study published in Neuron reveals new insights into our unparalleled sense of justice, specifically, the precise role of the dorsolateral prefrontal cortex (DLPFC)--one of the most recently evolved regions in the human brain. The findings reveal that DLPFC integrates information about a suspect's blameworthiness for wrongful acts and the resulting harm to others, enabling us to decide on the appropriate level of punishment.

"Despite the centrality of such third-party punishment decisions to modern institutions of justice, we don't know very much about how the brain combines evidence of intentionality and harm," says study first author Joshua Buckholtz of Harvard University. "Our study provides new insight into how humans make these judgments."

The success of our species is thought to rely largely on our capacity for large-scale cooperation; this, in turn, hinges on the uniquely human ability to establish and enforce social norms. To make decisions about how to punish those who violate these norms, it's necessary to integrate information about a suspect's culpability as well as the harmful consequences of the transgression. The DLPFC is well positioned to play this role; its cellular organization and high level of connectivity with other brain regions makes it specialized for integrating multiple streams of information in order to select appropriate responses. Others have shown that DLPFC performs this integrative role in non-social cognitive tasks, and the DLPFC appears to be activated in many studies of moral and legal norm-based decision-making. But until now, the precise role of DLPFC in making these judgments has been unclear.

To answer this question, Buckholtz and senior study author René Marois of Vanderbilt University used repetitive transcranial magnetic stimulation (rTMS)--a noninvasive way of stimulating the brain using magnetic fields --and functional magnetic resonance imaging (fMRI) in human subjects who made blameworthiness judgments and punishment decisions about a series of crime scenarios.

In each trial, subjects were shown a short written scenario describing a protagonist named John committing a crime, ranging from simple theft to assault and murder. In some cases, the crime was deliberate and John was fully responsible for his actions, but in other instances, his culpability was diminished due to duress, psychosis, or other mitigating factors. In separate sessions, subjects either rated John's blameworthiness or the severity of punishment he deserved.

The researchers first used rTMS to magnetically stimulate, and thereby temporarily disrupt, DLPFC activity in 66 healthy volunteers (half of whom received active rTMS; the other half received placebo or "sham" rTMS). DLPFC disruption reduced the level of punishment for wrongful acts without affecting blameworthiness ratings, suggesting that these two aspects of norm-based judgments rely on distinct cognitive and neurobiological processes. On closer inspection, the researchers found that rTMS only lowered punishment ratings when John's actions were deliberate but resulted in minimal harm. Further analysis revealed that DLPFC disruption caused subjects to base their punishment decisions more on the consequences of the crime rather than on John's intentions. The findings suggest that DLPFC plays a critical role in balancing information about intent and harm to enable appropriate punishment decisions.

A separate brain imaging experiment in the same study corroborated the main rTMS findings. Overall, DLPFC showed greater activity during punishment decisions compared to blameworthiness judgments. Moreover, DLPFC activation was sensitive to John's culpability level, but this effect was only found for punishment (not blameworthiness) judgments. The findings suggest that the DLPFC is not involved in assessing culpability per se; rather, this brain region uses information about culpability specifically to support punishment decision-making.

Taken together with past results, the findings suggest that the DLPFC receives relevant information about culpability and harmful consequences from other brain regions and then integrates this information to support punishment decision making. According to the authors, future studies should identify the precise computations involved in this integrative process.

In the meantime, the authors urge caution when it comes to interpreting the results, even though the findings suggest that a brief dose of magnetic stimulation could change how people make core legal judgments. "While this study does provide new insight into how human brains make decisions of the kind that judges and jurors make daily, the effects that we report are modest in size, and it's unclear how they would generalize to trial courts. The value of this study lies in its ability to reveal the basic mechanisms of norm-enforcement decisions," Marois says. "Magnetic brain stimulation will not be coming to a courtroom near you anytime soon.